Over the past two decades significant effort has been committed to the compression of digitized video signals for purposes of image storage and transmission. As a result many types of compression techniques have evolved including the use of discrete cosine transforms, sub-band encoding, pyramid transforms, intraframe encoding, interframe encoding and combinations of the above to name a few. More recently the International Organization for Standardization has developed a video compression standard for use in video storage applications, e.g., CD-ROM. This proposed standard is described in the document "Coding of Moving Pictures and Associated Audio", ISO-IEC JTC1/SC2/WG11, MPEG 90/176 Rev. 2; Dec. 18, 1990. Hereinbelow this system will be referred to as MPEG.
A feature of the MPEG standard is the use of both intraframe and interframe coding techniques in combination with discrete cosine transforms, run length encoding and statistical (Huffman) encoding. Intraframe encoding in general terms involves the encoding of an image frame from a single source frame to provide sufficient encoded data for reconstruction of an image from only the intraframe encoded data. Interframe encoding is the generation of encoded frame data from, for example, the differences between information from a current source frame and a frame predicted from prior frames. As such images may not be reconstructed from a frame of interframe encoded data without information from prior frames. The MPEG system incorporates two types of interframe encoding. The first develops predictive frames (designated P frames) from the current frame and a single prior frame. The second develops bidirectionally predictive frames (designated B frames) from the current frame and one or both of a prior and a subsequent frame. For example, assume that frames occur in a sequence F1, F2, F3, F4 . . . and that frame F1 is to be intraframe encoded (designated I frame), frames F2 and F3 are to be B frame encoded and frame F4 is to be P frame encoded. The P encoded frame is developed from differences between frame F4 and a predicted frame generated from a decoded version of I frame F1 only. The B encoded frame representing frame F2 (F3) is developed from differences between frame F2 (F3) and predicted frames generated from both a decoded version of I frame F1 and a decoded version of P frame F4. Exemplary circuitry for generating I, B and P encoded frames is described in "A Chip Set Core for Image Compression", by Alvin Artieri and Oswald Colavin, available from SGS-Thomson Microelectronics, Image Processing Business Unit, 17 avenue des Martyrs-B.P. 217, 38019 Grenoble Cedex France.
An exemplary sequence of I, B and P encoded frames is illustrated in FIG. 1A. In FIG. 1A the upper blocks correspond to odd fields of interlaced image data, and the lower blocks correspond to even fields of interlaced image data. The MPEG system protocol designates that only the odd fields of respective frames are to be encoded. The exemplary sequence includes 9 frames of I, B and P encoded data which sequences occur cyclically. The amount of encoded data of I frames is significantly greater than the amount of encoded data of P frames, and the amount of encoded data of B frames is less than that of encoded P frames. The number of P frames between I frames and the number of B frames between P or I and P frames is variable, i.e., it is user selectable within certain constraints. Nominally this selection is dependent upon the channel bandwidth and image content.
The level of encoding provided by the MPEG protocol (e.g., odd fields only and a continuous data transfer rate of 1.5M bits/S) is sufficient to produce acceptable images in the computer display environment. However, those skilled in the art of television signal processing will readily recognize that the MPEG protocol as defined will not provide images of current broadcast quality. It will also be recognized that minor modifications to the protocol will provide sufficient data to produce broadcast quality television images or even HDTV images. These changes include doubling the number of fields to be encoded as well as increasing the number of lines per field and the number of pixels per line. However, even with such modifications to the MPEG protocol, certain deficiencies will still exist to preclude acceptable performance with respect to image reception.
Regarding the TV environment, a first deficiency of the MPEG system is the timing latency of image production upon receiver turn on or channel change. An image cannot be reproduced until an intraframe encoded frame of data is available to the receiver. For the sequence of encoded frames shown in FIG. 1A, in the worst case, image reproduction has a latency of at least nine frame intervals. A second deficiency resides in the duration of image corruption due to corruption or loss of data in data transmission. That is, if data for an encoded I frame is lost or corrupted, the images reproduced during the succeeding eight frames will be in error, which error may become cumulatively worse over the interval.
Independently encoding the odd and even fields of image data such that intraframe encoded odd fields are located midway between intraframe encoded even fields can be utilized to significantly reduce the start-up interval and image reproduction during channel changes. In addition it provides signal information which may be utilized for signal error concealment.
The present invention is directed to apparatus for decompressing image data in which odd and even fields of image data have been independently encoded according to respective sequences of intra- and interframe compression modes.